In the Newtonian gauge, the perturbed FRW metric becomes [1]}, [2]}
\({\rm ds}^{2}=\biggl [-\left(1+2\phi \right){\rm d}{t}^{2}+{\mathrm {\bf a}}^{2}(t)\left(1-2\psi \right)\delta _{ij}d{x}^{i}d{x}^{j}\biggr ],\) 
